To modify the swivel force for the different body weights and preferences of different office chair users, various mechanism adjustment devices have already been proposed, however these mostly act on the preloading of springs.
If compression springs are used, an adjustment knob coaxial to the spring is operated. However, if torsion springs are used, the preload is varied by using elements which pull or push the free end of the spring.
The main drawback of this system is the fact that the preload adjustment cannot be too large otherwise insurmountable problems are introduced relating to spring reliability and chair design compactness. In addition to overcome the spring force during adjustment, an always critical compromise has to be reached between the physical force and the down-gearing (large number of screw turns) required for the adjustment. However the main drawback from the ergonomic viewpoint is that because the force increase during swiveling is always constant and if a “light” user encounters excessive hardness during swiveling, this for a “heavy” user could possibly be insufficient.
To obviate these drawbacks various solutions have already been proposed, such as those described in EP 1874161 or in U.S. Pat. No. 6,238,000, but these are costly and bulky.